Flow battery cleansing cycle to maintain electrolyte health and system performance

ABSTRACT

A method of cleansing a redox flow battery system may include operating the redox flow battery system in a charge, discharge, or idle mode, and responsive to a redox flow battery capacity being less than a threshold battery capacity, mixing the positive electrolyte with the negative electrolyte. In this way, battery capacity degradation following cyclic charging and discharging of the redox flow battery system can be substantially reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 62/491,964, entitled “Flow Battery Cleansing Cycle to MaintainElectrolyte Health and System Performance”, and filed on Apr. 28, 2017.The entire contents of the above-listed application are herebyincorporated by reference for all purposes.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under contract no.DEAR0000261 awarded by the DOE, Office of ARPA-E. The government hascertain rights in the invention.

FIELD

The present description relates generally to a method for operating aredox flow battery system.

BACKGROUND

Capacity degradation is a costly problem faced in the operation of allbatteries. Capacity degradation results, for example, from positive ornegative electrode side reactions, internal shorting, ionic movement, orthe like over a battery's lifetime. In the specific example of an ironredox flow battery, side reactions on the negative side include hydrogenevolution, as well as iron corrosion by proton (H⁺) and ferric (Fe³⁺)ions. Each of the side reactions adds to an imbalance in the positiveelectrolyte of the system, resulting in capacity degradation which cangrow over time and shorten the useful life of the battery.

In general, redox flow battery systems may have a lower rate of capacityloss compared to traditional batteries by using external (relative tothe battery cell) subsystems to manage positive and negative electrolytestates of health. For example, Evans (U.S. Pat. No. 9,509,011) disclosesa redox flow battery system with subsystems used to rebalance and managethe chemical states of positive electrolytes. In another example, Li(U.S. Patent Application 2016/0006054) discloses a redox flow systemwhere additional chemicals may be added to a redox flow battery systemto reduce the imbalance in the positive electrolyte.

The inventors herein have recognized potential disadvantages with themethods described above. Specifically, the addition of other chemicalsrequires separate tanks and an overall increase in system complexity andcost. Furthermore, due to subsystem inefficiencies, over time andrepeated cycles, electrolyte imbalance and therefore capacitydegradation may not be sufficiently mitigated.

In one example, the aforementioned issues may at least partially beaddressed by a method of operating a redox flow battery system,including, circulating a positive electrolyte between a positiveelectrode compartment and a positive electrolyte chamber with a positiveelectrolyte pump, circulating a negative electrolyte between a negativeelectrode compartment and a negative electrolyte chamber with a negativeelectrolyte pump, and responsive to a first condition, including when aredox flow battery capacity is less than a threshold battery capacity,performing a battery cleansing cycle, including mixing the positiveelectrolyte with the negative electrolyte until a redox flow batterystate of charge (SOC) is less than a threshold SOC.

In this way, the systems and methods described herein may maintainincreased redox flow battery system electrolyte health, includingreduced battery system capacity degradation caused by repeated andcyclic charging and discharging, as compared with conventional batterysystems. In particular, the systems and methods described herein enableoperation of redox flow battery systems for an increased number ofcycles without experiencing a loss of capacity greater than a thresholdcapacity loss. Furthermore, the methods and systems described herein maybe performed while utilizing existing electrolyte storage chambers, andwithout further additional electrolyte storage tanks, thereby reducing asystem complexity and cost.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an example of a redox flowbattery system having a multi-chamber electrolyte storage tank.

FIGS. 2 and 3 show example electrolyte flow circuit configurations ofthe redox flow battery system of FIG. 1 for performing a cleansingmethod for the redox flow battery system of FIG. 1.

FIG. 4 shows an example flowchart for a method of operating the redoxflow battery system of FIG. 1 in a cleansing mode.

FIG. 5 shows a graph of percent battery capacity vs. number of cyclesfor the redox flow battery system of FIG. 1.

DETAILED DESCRIPTION

An example electrolyte cleansing method for a redox flow battery systemis described herein. The cleansing method (or cleansing mode) may beemployed with a redox flow battery system, as described in FIG. 1, tomaintain electrolyte health and system performance. The method isdesigned for operation with redox flow battery systems where the sameelectrolyte chemistry is employed in both positive and negativeelectrolytes, resulting in systems that may operate for an unlimitednumber of recharge cycles without experiencing battery capacitydegradation. In at least one embodiment, the cleansing method maycomprise a control mechanism to determine the battery capacity anddirect the system implement a series of steps based upon comparing thedetermined battery capacity to a pre-set target.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by the deposit of one or more of the electro-activematerials as a solid layer on an electrode. Hybrid redox flow batteriesmay, for instance, include a chemical that plates via an electrochemicalreaction as a solid on a substrate throughout the battery chargeprocess. During battery discharge, the plated species may ionize via anelectrochemical reaction, becoming soluble in the electrolyte. In hybridbattery systems, the charge capacity (e.g., amount of energy stored) ofthe redox battery may be limited by the amount of metal plated duringbattery charge and may accordingly depend on the efficiency of theplating system as well as the available volume and surface areaavailable for plating.

In a redox flow battery system the negative electrode 26 may be referredto as the plating electrode and the positive electrode 28 may bereferred to as the redox electrode. The negative electrolyte within theplating side (e.g., negative electrode compartment 20) of the batterymay be referred to as the plating electrolyte and the positiveelectrolyte on the redox side (e.g. positive electrode compartment 22)of the battery may be referred to as the redox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Accordingly,during charge, the negative electrolyte and negative electrode may berespectively referred to as the catholyte and cathode of theelectrochemical reaction, while the positive electrolyte and thepositive electrode may be respectively referred to as the anolyte andanode of the electrochemical reaction. Alternatively, during discharge,the negative electrolyte and negative electrode may be respectivelyreferred to as the anolyte and anode of the electrochemical reaction,while the positive electrolyte and the positive electrode may berespectively referred to as the catholyte and cathode of theelectrochemical reaction. For simplicity, the terms positive andnegative are used herein to refer to the electrodes, electrolytes, andelectrode compartments in redox battery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode,ferrous ion, Fe²⁺, receives two electrons and plates as iron metal on tothe negative electrode 26 during battery charge, and iron metal, Fe⁰,loses two electrons and re-dissolves as Fe²⁺ during battery discharge.At the positive electrode, Fe²⁺ loses an electron to form ferric ion,Fe³⁺, during charge, and during discharge Fe³⁺ gains an electron to formFe²⁺. The electrochemical reaction is summarized in equations (1) and(2), wherein the forward reactions (left to right) indicateelectrochemical reactions during battery charge, while the reversereactions (right to left) indicate electrochemical reactions duringbattery discharge:

$\begin{matrix}\begin{matrix}\left. {{Fe}^{2 +} + {2e^{-}}}\leftrightarrow{Fe}^{0} \right. & {{- 0.44}\mspace{14mu} V} & \left( {{Negative}\mspace{14mu}{Electrode}} \right)\end{matrix} & (1) \\\begin{matrix}\left. {Fe}^{2 +}\leftrightarrow{{2{Fe}^{3 +}} + {2e^{-}}} \right. & {{+ 0.77}\mspace{14mu} V} & \left( {{Positive}{\mspace{11mu}\;}{Electrode}} \right)\end{matrix} & (2)\end{matrix}$

As discussed above, the negative electrolyte used in the all iron redoxflow battery (IFB) may provide a sufficient amount of Fe²⁺ so that,during charge, Fe²⁺ can accept two electrons from the negative electrodeto form Fe⁰ and plate onto a substrate. During discharge, the plated Fe⁰may then lose two electrons, ionizing into Fe²⁺ and be dissolved backinto the electrolyte. The equilibrium potential of the above reaction is−0.44V and thus this reaction provides a negative terminal for thedesired system. On the positive side of the IFB, the electrolyte mayprovide Fe²⁺ during charge which loses electron and oxidizes to Fe³⁺.During discharge, Fe³⁺ provided by the electrolyte becomes Fe²⁺ byabsorbing an electron provided by the electrode. The equilibriumpotential of this reaction is +0.77V, creating a positive terminal forthe desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode may be coupled via terminal40 to the negative side of a voltage source so that electrons may bedelivered to the negative electrolyte via the positive electrode (e.g.,as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in the positiveelectrode compartment 22). The electrons provided to the negativeelectrode 26 (e.g., plating electrode) can reduce the Fe²⁺ in thenegative electrolyte to form Fe⁰ at the plating substrate causing it toplate onto the negative electrode.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofcell 18 to provide additional Fe³⁺ ions via an external source, such asan external positive electrolyte chamber or positive electrolyte chamber52. More commonly, availability of Fe⁰ during discharge may be an issuein IFB systems, wherein the Fe⁰ available for discharge may beproportional to the surface area and volume of the negative electrodesubstrate as well as the plating efficiency. Charge capacity may bedependent on the availability of Fe²⁺ in the negative electrodecompartment 20. As an example, Fe²⁺ availability can be maintained byproviding additional Fe²⁺ ions via an external source, such as anexternal negative electrolyte chamber 50 to increase the concentrationor the volume of the negative electrolyte to the negative electrodecompartment 20 side of cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough the separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) can result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ can damage the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, adding to process cost and complexity.Adding specific organic acids to the positive electrolyte and thenegative electrolyte in response to electrolyte pH changes may alsomitigate precipitate formation during battery charge and dischargecycling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

During charge of an IFB, for example, ferrous ion, Fe²⁺, is reduced(accepts two electrons in a redox reaction) to metal iron, Fe⁰, at thenegative electrode. Simultaneously, at the positive electrode, ferrousion, Fe²⁺, is oxidized (loss of an electron) to ferric ion, Fe³⁺.Concurrently, at the negative electrode, the ferrous iron reductionreaction competes with the reduction of protons, H⁺, wherein two protonseach accept a single electron to form hydrogen gas, H₂ and the corrosionof iron metal to produce ferrous ion, Fe²⁺. The production of hydrogengas through reduction of hydrogen protons and the corrosion of ironmetal are shown in equations (3) and (4), respectively:

$\begin{matrix}\begin{matrix}\left. {H^{+} + e^{-}}\leftrightarrow{\frac{1}{2}H_{2}} \right. & \left( {{proton}\mspace{14mu}{reduction}} \right)\end{matrix} & (3) \\\begin{matrix}\left. {{Fe}^{0} + {2H^{+}}}\leftrightarrow{{Fe}^{2 +} + H_{2}} \right. & \left( {{iron}\mspace{14mu}{corrosion}} \right)\end{matrix} & (4)\end{matrix}$

As a result, the negative electrolyte in the negative electrodecompartment 20 tends to stabilize at a pH range between 3 and 6. At thepositive electrode compartment 22, ferric ion, Fe³⁺, has a much loweracid disassociation constant (pKa) than that of ferrous ion, Fe²⁺.Therefore, as more ferrous ions are oxidized to ferric ions, thepositive electrolyte tends to stabilize at a pH less than 2, inparticular at a pH closer to 1.

Accordingly, maintaining the positive electrolyte pH in a first range inwhich the positive electrolyte (positive electrode compartment 22)remains stable and maintaining the negative electrolyte pH in a secondrange in which the negative electrolyte (negative electrode compartment20) remains stable may reduce low cycling performance and increaseefficiency of redox flow batteries. For example, maintaining a pH of anegative electrolyte in an IFB between 3 and 4 may reduce iron corrosionreactions and increase iron plating efficiency, while maintaining a pHof a positive electrolyte less than 2, in particular less than 1, maypromote the ferric/ferrous ion redox reaction and reduce ferrichydroxide formation.

As indicated by equation (3) and (4), evolution of hydrogen can causeelectrolyte imbalance in a redox flow battery system. For example,during charge, electrons flowing from the positive electrode to thenegative electrode (e.g., as a result of ferrous ion oxidation), may beconsumed by hydrogen evolution via equation (3), thereby reducing theelectrons available for plating given by equation (1). Because of thereduced plating, battery charge capacity is reduced. Additionally,corrosion of the iron metal further reduces battery capacity since adecreased amount of iron metal is available for battery discharge. Thus,an imbalanced electrolyte state of charge between the positive electrodecompartment 22 and the negative electrode compartment 20 can develop asa result of hydrogen production via reaction (3) and (4). Furthermore,hydrogen gas production resulting from iron metal corrosion and protonreduction both consume protons, which can result in a pH increase of thenegative electrolyte. As discussed above with reference to FIG. 1, anincrease in pH may destabilize the electrolyte in the redox batter flowsystem, resulting in further battery capacity and efficiency losses.

An approach that addresses the electrolyte rebalancing issues that canbe caused by hydrogen gas production in redox flow battery systemscomprises reducing the imbalanced ion in the positive electrolyte withhydrogen generated from the side reactions. As an example, in an IFBsystem, the positive electrolyte comprising ferric ion may be reduced bythe hydrogen gas according to equation (5):

$\begin{matrix}\left. {{Fe}^{3 +} + {{1/2}H_{2}}}\rightarrow{{Fe}^{2 +} + H^{+}} \right. & (5)\end{matrix}$

In the IFB system example, by reacting ferric ion with hydrogen gas, thehydrogen gas can be converted back to protons, thereby maintain asubstantially constant pH in the negative electrode compartment 20 andthe positive electrode compartment 22. Furthermore, by converting ferricion to ferrous ion, the state of charge of the positive electrolyte inthe positive electrode compartment 22 may be rebalanced with the stateof charge of the negative electrolyte in the negative electrodecompartment 20. Although equation (5) is written for rebalancingelectrolytes in an IFB system, the method of reducing an electrolytewith hydrogen gas may be generalized by equation (6):

$\begin{matrix}\left. {M^{x +} + {\frac{\left( {x - z} \right)}{2}H_{2}}}\rightarrow{M^{z +} + {\left( {x - z} \right)H^{+}}} \right. & (6)\end{matrix}$

In equation (6), M^(x+) represents the positive electrolyte M havingionic charge, x, M^(z+) represents the reduced electrolyte M havingionic charge, z.

A catalyst comprising graphite or comprising supported precious metal(e.g., carbon-supported Pt, Rd, Ru, or alloys thereof) catalyst mayincrease the rate of reaction described by equation (5) for practicalutilization in a redox flow battery system. As an example, hydrogen gasgenerated in the redox flow battery system may be directed to a catalystsurface, and hydrogen gas and electrolyte (e.g., comprising ferric ion)may be fluidly contacted at the catalyst surface, wherein the hydrogengas chemically reduces the ferric ion to ferrous ion and producespositive hydrogen ions (e.g., protons).

FIG. 1 is a schematic drawing of a redox flow battery system. FIGS. 2-3illustrate various perspective views of electrolyte storage chambers,including the electrolyte storage chambers shown in FIG. 1 and that maybe coupled to the redox flow battery system in FIG. 1. FIG. 4illustrates a flow diagram of a cleansing method that may be coupled toand implemented in the operation of the redox flow battery system inFIG. 1. FIG. 5 illustrates a graph of percent battery capacity versusnumber of cycles, for both a redox iron flow battery as in FIG. 1 and alithium ion battery.

FIG. 1 provides a schematic illustration of a redox flow battery system10. The redox flow battery system 10 may comprise a redox flow batterycell 18, fluidly connected to a multi-chambered electrolyte storage tank110. The redox flow battery cell 18 may generally include a negativeelectrode compartment 20, separator 24, and positive electrodecompartment 22. The separator 24 may comprise an electrically insulatingionic conducting barrier which prevents bulk mixing of the positiveelectrolyte and the negative electrolyte while allowing conductance ofspecific ions therethrough. For example, the separator 24 may comprisean ion-exchange membrane and/or a microporous membrane. The negativeelectrode compartment 20 may comprise a negative electrode 26, and anegative electrolyte comprising electroactive materials. The positiveelectrode compartment 22 may comprise a positive electrode 28, and apositive electrolyte comprising electroactive materials. In someexamples, multiple redox flow battery cells 18 may be combined in seriesor parallel to generate a higher voltage or current in a redox flowbattery system. Further illustrated in FIG. 1 are negative and positiveelectrolyte pumps 30 and 32, both used to pump electrolyte solutionthrough the flow battery system 10. Electrolytes are stored in one ormore tanks external to the cell, and are pumped via negative andpositive electrolyte pumps 30 and 32 through the negative electrodecompartment 20 side and the positive electrode compartment 22 side ofthe battery, respectively.

As illustrated in FIG. 1, the redox flow battery cell 18 may furtherinclude negative battery terminal 40, and positive battery terminal 42.When a charge current is applied to the battery terminals 40 and 42, thepositive electrolyte is oxidized (lose one or more electrons) at thepositive electrode 28, and the negative electrolyte is reduced (gain oneor more electrons) at the negative electrode 26. During batterydischarge, reverse redox reactions occur on the electrodes. In otherwords, the positive electrolyte is reduced (gain one or more electrons)at the positive electrode 28, and the negative electrolyte is oxidized(lose one or more electrons) at the negative electrode 26. Theelectrical potential difference across the battery is maintained by theelectrochemical redox reactions in the positive electrode compartment 22and the negative electrode compartment 20, and can induce a currentthrough a conductor while the reactions are sustained. The amount ofenergy stored by a redox battery is limited by the amount ofelectro-active material available in electrolytes for discharge,depending on the total volume of electrolytes and the solubility of theelectro-active materials.

The flow battery system 10 may further comprise an integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber 50 holds negative electrolytecomprising electroactive materials, and the positive electrolyte chamber52 holds positive electrolyte comprising electroactive materials. Thebulkhead 98 may be positioned within the multi-chambered storage tank110 to yield a desired volume ratio between the negative electrolytechamber 50 and the positive electrolyte chamber 52. In one example, thebulkhead 98 may be positioned to set the volume ratio of the negativeand positive electrolyte chambers according to the stoichiometric ratiobetween the negative and positive redox reactions. The figure furtherillustrates the fill height 112 of storage tank 110, which may indicatethe liquid level in each tank compartment. The figure also shows gashead space 90 located above the fill height 112 of negative electrolytechamber 50, and gas head space 92 located above the fill height 112 ofpositive electrolyte chamber 52. The gas head space 92 may be utilizedto store hydrogen gas generated through operation of the redox flowbattery (e.g., due to proton reduction and corrosion side reactions) andconveyed to the multi-chambered storage tank 110 with returningelectrolyte from the redox flow battery cell 18. The hydrogen gas may beseparated spontaneously at the gas-liquid interface (e.g., fill height112) within the multi-chambered storage tank 110, thereby precludinghaving additional gas-liquid separators as part of the redox flowbattery system. Once separated from the electrolyte, the hydrogen gasmay fill the gas head spaces 90 and 92. A such, the stored hydrogen gascan aid in purging other gases from the multi-chambered electrolytestorage tank 110, thereby acting as an inert gas blanket for reducingoxidation of electrolyte species, which can help to reduce redox flowbattery capacity losses. In this way, utilizing the integratedmulti-chambered storage tank 110 may forego having separate negative andpositive electrolyte storage tanks, hydrogen storage tanks, andgas-liquid separators common to conventional redox flow battery systems,thereby simplifying the system design, reducing the physical footprintof the system, and reducing system costs.

FIG. 1 also shows the spill over-hole 96, which creates an opening inthe bulkhead 98 between gas head spaces 90 and 92, and provides a meansof equalizing gas pressure between the two chambers. The spill over hole96 may be positioned at a threshold height above the fill height 112.The spill over hole further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe²⁺) is used in bothnegative and positive electrode compartments 20 and 22, so spilling overof electrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank can include at least one outlet from eachof the negative and positive electrolyte chambers, and at least oneinlet to each of the negative and positive electrolyte chambers.Furthermore, one or more outlet connections may be provided from the gashead spaces 90 and 92 for directing hydrogen gas to rebalancing reactors80 and 82.

Although not shown in FIG. 1, integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by the controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber independently or together. For example, in responseto an electrolyte temperature decreasing below a threshold temperature,the controller may increase a power supplied to one or more heaters sothat a heat flux to the electrolyte is increased. The electrolytetemperature may be indicated by one or more temperature sensors mountedat the multi-chambered electrolyte storage tank 110, including sensors60 and 62. As examples the one or more heaters may include coil typeheaters or other immersion heaters immersed in the electrolyte fluid, orsurface mantle type heaters that transfer heat conductively through thewalls of the negative and positive electrolyte chambers to heat thefluid therein. Other known types of tank heaters may be employed withoutdeparting from the scope of the present disclosure. Furthermore,controller 88 may deactivate one or more heaters in the negative andpositive electrolyte chambers in response to a liquid level decreasingbelow a solids fill threshold level. Said in another way, controller 88may activate the one or more heaters in the negative and positiveelectrolyte chambers only in response to a liquid level increasing abovethe solids fill threshold level. In this way, activating the one or moreheaters without sufficient liquid in the positive and/or negativeelectrolyte chambers can be averted, thereby reducing a risk ofoverheating or burning out the heaters.

Further illustrated in FIG. 1, electrolyte solutions typically stored inthe multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82, may be connected in-lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the battery, respectively, in theredox flow battery system 10. One or more rebalancing reactors may beconnected in-line with the recirculating flow paths of the electrolyteat the negative and positive sides of the battery, and other rebalancingreactors may be connected in parallel, for redundancy (e.g., arebalancing reactor may be serviced without disrupting battery andrebalancing operations) and for increased rebalancing capacity. In oneexample, the electrolyte rebalancing reactors 80 and 82 may be placed inthe return flow path from the positive and negative electrodecompartments 20 and 22 to the positive and negative electrolyte chambers50 and 52, respectively. Electrolyte rebalancing reactors 80 and 82 mayserve to rebalance electrolyte charge imbalances in the redox flowbattery system occurring due to side reactions, ion crossover, and thelike, as described herein. In one example, electrolyte rebalancingreactors 80 and 82 may include trickle bed reactors, where the hydrogengas and electrolyte are contacted at catalyst surfaces in a packed bedfor carrying out the electrolyte rebalancing reaction. In other examplesthe rebalancing reactors 80 and 82 may include flow-through typereactors that are capable of contacting the hydrogen gas and theelectrolyte liquid and carrying out the rebalancing reactions in theabsence a packed catalyst bed.

During operation of a redox flow battery system, sensors and probes maymonitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1, sensors 62 and 60 maybe be positionedto monitor positive electrolyte and negative electrolyte conditions atthe positive electrolyte chamber 52 and the negative electrolyte chamber50, respectively. As another example, sensors 72 and 70, alsoillustrated in FIG. 1, may monitor positive electrolyte and negativeelectrolyte conditions at the positive electrode compartment 22 and thenegative electrode compartment 20, respectively. In one example, sensors70 and 72 may include pressure sensors that transmit signals to thecontroller 88 indicating the pressure at the negative and positive sidesof the separator 24 of the redox flow battery cell 18. The pressure atthe negative and positive electrode compartments 20 and 22 of theseparator 24 may be regulated by controlling the inlet and outlet flowsof negative and positive electrolyte thereto, respectively. For example,the controller may decrease a pressure at the negative electrodecompartment 20 by one or more of increasing a pump speed of a vacuumpump fluidly coupled to thereto, reducing a pump speed of the negativeelectrolyte pump 30, and by throttling a back pressure flow regulator toincrease an outlet flow from the negative electrode compartment.

Similarly, the controller may increase a pressure at the positiveelectrode compartment 22 by one or more of increasing a pump speed ofthe positive electrolyte pump 32, and by throttling a back pressure flowregulator to decrease an outlet flow from the negative electrodecompartment. Back pressure flow regulators may include orifices, valves,and the like. For example, controller 88 may send a signal to position avalve to a more open position, to induce higher outlet flows fromnegative electrode compartment 20, thereby reducing a negative electrodecompartment pressure. Increasing the positive electrode compartmentpressure and decreasing the pressure in the negative electrodecompartment may aid in increasing a cross-over pressure (positive overnegative) across the separator 24. Increasing the cross-over pressure byincreasing the flow of the positive electrolyte by increasing the pumpspeed of the positive electrolyte pump 32 and increasing back pressureat the outlet of the positive electrode compartment 22 may be lessdesirable than other methods of increasing the cross-over pressurebecause pump parasitic losses may be increased.

Sensors may be positioned at other locations throughout the redox flowbattery system to monitor electrolyte chemical properties and otherproperties. For example a sensor may be positioned in an external acidtank (not shown) to monitor acid volume or pH of the external acid tank,wherein acid from the external acid tank is supplied via an externalpump (not shown) to the redox flow battery system in order to reduceprecipitate formation in the electrolytes. Additional external tanks andsensors may be installed for supplying other additives to the redox flowbattery system 10. For example, various sensor including, temperature,pressure, conductivity, and level sensors of a field hydration systemmay transmit signals to the controller 88 when hydrating a redox flowbattery system in a dry state. Furthermore, controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system. Sensorinformation may be transmitted to a controller 88 which may in turnactuate negative and positive electrolyte pumps 30 and 32 to controlelectrolyte flow through the cell 18, or to perform other controlfunctions, as an example. In this manner, the controller 88 may beresponsive to, one or a combination of sensors and probes. Redox flowbattery cell 18 may be positioned within one of a plurality of redoxflow battery cell stacks of a power module for a redox flow batterysystem. Each of the redox flow battery cells 18 in a redox flow batterycell stack may be electrically connected in series and/or parallel witha plurality of other redox flow battery cells in the redox flow batterycell stack. Furthermore each of the redox flow battery cell stacks maybe electrically connected in series and/or parallel with a plurality ofthe other redox flow battery cell stacks in the power module. In thisway, the redox flow battery cell stacks may be electrically combined tosupply power from the power module.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1, hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110.

The integrated multi-chambered electrolyte storage tank 110 maysupplement the hydrogen gas generated in redox flow battery system 10.For example, when gas leaks are detected in redox flow battery system 10or when the reduction reaction rate is too low at low hydrogen partialpressure, hydrogen gas may be supplied from the integratedmulti-chambered electrolyte storage tank 110 in order to rebalance thestate of charge of the electro-active species in the positiveelectrolyte and negative electrolyte. As an example, controller 88 maysupply hydrogen gas from integrated multi-chambered electrolyte storagetank 110 in response to a measured change in pH or in response to ameasured change in state of charge of an electrolyte or anelectro-active species. For example an increase in pH of the negativeelectrolyte chamber 50, or the negative electrode compartment 20, mayindicate that hydrogen is leaking from the redox flow battery system 10and/or that the reaction rate is too slow with the available hydrogenpartial pressure. In response to the pH increase, controller 88 mayincrease a supply of hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 to the redox flow battery system 10. As afurther example, controller 88 may supply hydrogen gas from integratedmulti-chambered electrolyte storage tank 110 in response to a pH change,wherein the pH increases beyond a first threshold pH or decreases beyonda second threshold pH. In the case of an IFB, controller 88 may supplyadditional hydrogen to increase the rate of reduction of ferric ions andthe rate of production of protons, thereby reducing the pH of thepositive electrolyte. Furthermore, the negative electrolyte pH may belowered by hydrogen reduction of ferric ions crossing over from thepositive electrolyte to the negative electrolyte or by proton generatedat the positive side crossing over to the negative electrolyte due to aproton concentration gradient and electrophoretic forces. In thismanner, the pH of the negative electrolyte may be maintained within astable region, while reducing the risk of precipitation of ferric ions(crossing over from the positive electrode compartment) as Fe(OH)3.

Other control schemes for controlling the supply rate of hydrogen gasfrom integrated multi-chambered electrolyte storage tank 110 responsiveto a change in an electrolyte pH or to a change in an electrolyte stateof charge, detected by other sensors such as an oxygen-reductionpotential (ORP) meter or an optical sensor, may be implemented. Furtherstill, the change in pH or state of charge triggering the action ofcontroller 88 may be based on a rate of change or a change measured overa time period. The time period for the rate of change may bepredetermined or adjusted based on the time constants for the redox flowbattery system. For example the time period may be reduced if therecirculation rate is high, and local changes in concentration (e.g.,due to side reactions or gas leaks) may quickly be measured since thetime constants may be small.

FIGS. 2 and 3 present example electrolyte flow circuit configurations,200 and 300 respectively, by which electrolyte storage chambers may befluidly connected for operation of the cleansing method disclosed hereinfor cleansing the redox flow battery system of FIG. 1. The redox flowbattery system 10 may include one of the electrolyte flow circuitconfigurations 200 and 300. In FIG. 2, positive electrolyte chamber 52and negative electrolyte chamber 50 may be coupled with a first mixingvalve 210 and an orifice 220. More specifically, an inlet to the firstmixing valve 210 may be fluidly coupled within a fluid passage 292positioned to divert electrolyte from the second electrolyte circuit 282at a discharge from the positive electrolyte chamber 52 upstream of thepositive electrolyte pump 32; the outlet of the first mixing valve 210may be fluidly coupled to the first electrolyte circuit 280 downstreamof the negative electrolyte pump 30. Furthermore, an inlet to theorifice 220 may be fluidly coupled within a fluid passage 290 positionedto divert electrolyte from the first electrolyte circuit 280 at adischarge from the negative electrolyte chamber 50 upstream of thenegative electrolyte pump 30; the outlet of the orifice 220 may befluidly coupled to the second electrolyte circuit 282 downstream of thepositive electrolyte pump 32.

In FIG. 3, a flow circuit configuration 300 includes positiveelectrolyte chamber 52 and negative electrolyte chamber 50, which arefluidly connected with first and second motor-controlled mixing valves210 and 310, respectively. As in the flow circuit configuration 200 ofFIG. 2, an inlet to the first mixing valve 210 may be fluidly coupledwithin a fluid passage 292 positioned to divert electrolyte from thesecond electrolyte circuit 282 at a discharge from the positiveelectrolyte chamber 52 upstream of the positive electrolyte pump 32; theoutlet of the first mixing valve 210 may be fluidly coupled to the firstelectrolyte circuit 280 downstream of the negative electrolyte pump 30.Furthermore, an inlet to the mixing valve 310 may be fluidly coupledwithin a fluid passage 290 positioned to divert electrolyte from thefirst electrolyte circuit 280 at a discharge from the negativeelectrolyte chamber 50 upstream of the negative electrolyte pump 30; theoutlet of the mixing valve 310 may be fluidly coupled to the secondelectrolyte circuit 282 downstream of the positive electrolyte pump 32.

In both FIGS. 2 and 3, the motor-controlled mixing valves 210 and 310may be actuated by a controller, such as controller 88 of the redox flowbattery system, and the positive and negative electrolyte chambers 52and 50 are further fluidly connected to the redox flow battery cell 18.The flow configurations illustrated in FIGS. 2 and 3 may be generalizedand applied to multiple tank or multi-chamber single tank electrolytestorage configurations. For example, positive electrolyte chamber 52 andnegative electrolyte chamber 50 may correspond to the positiveelectrolyte chamber 52 and negative electrolyte chamber 50 of themulti-chamber electrolyte storage tank 110 of FIG. 1.

The negative electrolyte chamber 50, head space 90, and redox flowbattery cell 18 including a negative electrode compartment 20, arearranged along a first electrolyte circuit 280. Furthermore, althoughnot shown in FIG. 2, a first rebalancing reactor (e.g., rebalancingreactor 80 of FIG. 1) may also be arranged in series along the firstelectrolyte circuit 280 or in parallel thereto. During batterydischarge, charge, and idle modes, negative electrolytes from thenegative electrolyte chamber 50 may flow through the first electrolytecircuit 280 with assistance from the negative electrolyte pump 30. Thus,when the negative electrolyte pump 30 is active (e.g., ON), negativeelectrolytes flow through the first electrolyte circuit 280.

The positive electrolyte chamber 52, head space 92, and redox flowbattery cell 18 including positive electrode compartment 22, arearranged along a second electrolyte circuit 282. Furthermore, a secondrebalancing reactor (e.g., rebalancing reactor 82 of FIG. 1) may also bearranged in series along the second electrolyte circuit 282 or inparallel thereto. During battery discharge, charge, and idle modes,positive electrolytes from the positive electrolyte chamber 52 may flowthrough the second electrolyte circuit 282 with assistance from thepositive electrolyte pump 32. Thus, when the positive electrolyte pump32 is active (e.g., ON), positive electrolytes flow through the secondelectrolyte circuit 282.

The first 280 and second 282 electrolyte circuits are fluidly separatedfrom one another outside of during a cleansing mode of operation of theredox flow battery system. In this way, negative electrolytes from thenegative electrolyte chamber 50 in the first electrolyte circuit 280 donot mix with positive electrolyte from the positive electrolyte chamber52 in the second electrolyte circuit 282 outside of the cleansing modeduring charge, discharge, and idle modes.

The cleansing mode may be activated in response to one or more of asystem available capacity being lower than a threshold capacity at agiven battery state of charge (SOC) and battery charge/dischargeperformance being lower than a threshold performance at a given batterySOC. The battery charge/discharge performance can include acharge/discharge current and/or charge/discharge voltage, respectively.Similarly, the threshold performance may refer to a threshold chargecurrent, a threshold discharge current, a threshold charge voltage, athreshold charge current, or a combination thereof. In other words, abattery charge performance may be lower than a threshold chargeperformance at a given battery SOC when the battery charge voltage islower than a threshold charge voltage and/or when the battery chargecurrent is less than a threshold charge current. This may occur when anelectrolytic imbalance occurs between the negative and positiveelectrolyte chambers 50 and 52. The first electrolyte circuit 280 andthe second electrolyte circuit 282 may be fluidly coupled during thecleansing mode. As such, electrolytes from the positive electrolytechamber 52 may flow into the first electrolyte circuit 280 andelectrolytes from the negative electrolyte chamber may flow into thesecond electrolyte circuit 282.

In the embodiment of FIG. 2, the first 280 and second 282 electrolytecircuits may be fluidly coupled via one or more of the first mixingvalve 210 and the orifice 220. The first mixing valve 210 may allowelectrolytes from the positive electrolyte chamber 52 of the secondelectrolyte circuit 282 to flow into the first electrolyte circuit 280.In one example, electrolytes from the negative electrolyte chamber 50 donot flow from the first electrolyte circuit 280 to the secondelectrolyte circuit 282 via the first mixing valve 210.

The orifice 220 may allow electrolytes from the negative electrolytechamber 50 of the first electrolyte circuit 280 to flow into the secondelectrolyte circuit 282. In one example, electrolytes from the positiveelectrolyte chamber 52 do not flow from the second electrolyte circuit282 to the first electrolyte circuit 280 via the orifice 220.

The orifice 220 may be sized to adjust a flow rate from the firstelectrolyte circuit 280 to the second electrolyte circuit 282. Theorifice 220 may be sized based on a flow rate of the negativeelectrolyte pump 30. In one example, the orifice 220 is sized to providea flow rate between the first 280 and second 282 electrolyte circuitssubstantially equal to a flow rate of the negative electrolyte pump 30.In one example, a mixing rate between the first 280 and second 282electrolyte circuits increases as a size of the orifice 220 increases.

In some embodiments, additionally or alternatively, the orifice 220fluidly couples the first 280 and second 282 electrolyte circuitsoutside of the cleansing mode. As such, electrolyte flow between thefirst 280 and second 282 electrolyte circuits may be based on a pressuredifference between the circuits. For example, if a pressure of the firstelectrolyte circuit 280 is greater than a pressure of the secondelectrolyte circuit 282, then the orifice 220 may permit electrolytes toflow from the first circuit 280 to the second circuit 282.

When one or more of the system available capacity is lower than thethreshold capacity at a given battery state of charge (SOC) and/or thebattery charge/discharge performance is lower than a thresholdperformance at a given battery SOC, the cleansing mode and the dischargemode are initiated. As shown above in equations 1 and 2, compositions ofthe negative and positive electrolyte chambers 50 and 52 aresubstantially similar during the discharge mode. As such, systemcontroller 88 may initiate start of the cleansing cycle, which entailsdischarging the battery system beyond a threshold discharge SOC,including positive and negative electrolyte flowing through therebalancing reactors 80 and 82 to reduce charge imbalances between thepositive and negative sides of the battery. For example, when thepositive electrolyte pH is less than a threshold discharge pH andpositive electrolyte SOC is lower than a threshold discharge SOC, thenthe start of the cleansing cycle may be triggered. The thresholddischarge pH may be selected to reduce a risk of Fe(OH)₃ precipitationresulting from mixing the positive electrolyte (having a lower pH) andnegative electrolyte (having a higher pH). The threshold discharge SOCmay be selected to reduce an amount of Ferric ion being introduced intothe negative electrolyte.

During the cleansing mode, electrolyte may initially flow from thesecond electrolyte circuit 282 to the first electrolyte circuit 280.This may include opening the first mixing valve 210 and activating thenegative electrolyte pump 30 to draw electrolyte from the secondelectrolyte circuit 282 to the first electrolyte circuit 280.Furthermore, the positive electrolyte pump 32 may be deactivated. Assuch, electrolytes may not be circulated by the positive electrolytepump 32 through the second electrolyte circuit 282. In this way,electrolytes from the second electrolyte circuit 282 mix withelectrolytes in the first electrolyte circuit 280 when the first mixingvalve 210 is open and the negative electrolyte pump 30 is activated.Conversely, while the positive electrolyte pump 32 is deactivated,circulating electrolytes in the first electrolyte circuit 280 may notflow to the second electrolyte circuit 282. By utilizing the negativeelectrolyte pump 30 to circulate electrolytes through the firstelectrolyte circuit 280 during the cleansing cycle, an additional pumpto the negative electrolyte pump 30 and the positive electrolyte pump 32for cleansing the electrolyte chambers may not be included in the redoxflow battery system (e.g., redox flow battery system 10 of FIG. 1). Assuch, packaging restraints and manufacturing costs are reduced comparedto systems having additional pumps other than the negative electrolytepump 30 and the positive electrolyte pump 32, which are already utilizedto circulate electrolyte during charging and discharging of the redoxflow battery system 10.

Electrolytes from the second electrolyte circuit 282 may flow to thefirst electrolyte circuit 280 for a first threshold duration. The firstthreshold duration may be fixed. Alternatively, the first thresholdduration may be adjusted based on a difference between the battery SOCand the threshold SOC. In one example, the first threshold durationincreases as the difference between the battery SOC and the thresholdSOC increases and as a result, a greater amount of electrolytes from thesecond electrolyte circuit 282 flow to the first electrolyte circuit280.

Responsive to the first threshold duration elapsing, the first mixingvalve 210 is closed, while the negative electrolyte pump 30 remainsactive. As such, electrolytes from both the first 280 and second 282electrolyte circuits continue to flow through the first electrolytecircuit 280 without flowing to the second electrolyte circuit 282. Thismay occur for a time delay. In one example, the time delay is fixed.Alternatively, the time delay may be proportionally related to the firstthreshold duration, wherein as the first threshold duration increases,the time delay increases. In this way, electrolytes flow through onlythe first electrolyte circuit 280 during the time delay. Flowingelectrolytes through only the first electrolyte circuit 280 during thetime delay aids in more thorough mixing of the electrolytes in the firstelectrolyte circuit 280 during the time delay when no furtherelectrolyte flows from the second electrolyte circuit 282 into the firstelectrolyte circuit 280.

Responsive to elapsing of the time delay, the negative electrolyte pump30 is deactivated, and the positive electrolyte pump 32 is activated,thereby promoting flow of electrolytes from the first electrolytecircuit 280 to the second electrolyte circuit 282 by way of the orifice220. Electrolytes may flow from the first electrolyte circuit 280 to thesecond electrolyte circuit 282 for a second threshold duration. In oneexample, the second threshold duration is exactly equal to the firstthreshold duration. Alternatively, the second threshold duration may bebased on a combination of a sizing of the orifice and the firstthreshold duration. At any rate, the second threshold duration may be anamount of time desired to allow electrolytes to flow from the firstelectrolyte circuit 280 to the second electrolyte circuit 282 to balancean electrolyte volume in both of the first and second electrolytecircuits 280 and 282.

Additionally or alternatively, the cleansing cycle may be activated andelectrolytes may flow from the first electrolyte circuit 280 to thesecond electrolyte circuit only when a battery SOC is less than thethreshold discharge SOC. In response to the battery SOC being greaterthan the threshold discharge SOC, the cleansing mode may be deactivatedand the battery system may return to operating in the charge, discharge,or idle mode. In one example, deactivating the cleansing mode includesclosing the first mixing valve 210 and preventing flow of electrolytesbetween the first 280 and second 282 electrolyte circuits.

In the embodiment of FIG. 3, the first and second mixing valves 210 and310, respectively, may be electrically, hydraulically, and/orpneumatically powered, and electrically coupled to the controller 88.The positive and negative electrolyte chambers 52 and 50 may enter acleansing mode in response to a state of charge (SOC) of the redox flowbattery cell 18 falling below a threshold SOC and an electrolyte pHfalling below a threshold pH. For example, when the positive electrolytepH is less than a threshold discharge pH and positive electrolyte SOC islower than a threshold discharge SOC, then the start of the cleansingcycle may be triggered. The threshold discharge pH may be selected toreduce a risk of Fe(OH)₃ precipitation resulting from mixing thepositive electrolyte (having a lower pH) and negative electrolyte(having a higher pH). The threshold discharge SOC may be selected toreduce an amount of Ferric ion being introduced into the negativeelectrolyte.

The second mixing valve 310 is arranged in a location of the batterysystem similar to a location of the orifice 220 of FIG. 2. The secondmixing valve 310 may be positioned to prevent first 280 and second 282electrolyte circuits from mixing during operation outside of thecleansing mode. In this way, the configuration of FIG. 3, includingfirst and second mixing valves 210 and 310, may be advantageous over theconfiguration of FIG. 2, including first mixing valve 210 and orifice220. However, the configuration of FIG. 2 may be less costly and simplerto implement.

For example, when the first mixing valve 210 is open and the negativeelectrolyte pump 30 is activated to promote the flow of electrolytesfrom the second electrolyte circuit 282 to the first electrolyte circuit280, the second mixing valve 310 is moved to a closed position. In thisway, the positive electrolyte pump 32 may be activated while electrolyteflow from the second circuit 282 to the first circuit 280. In this way,a portion of electrolytes in the second circuit 282 may remain andcirculate through the second circuit 282, while a remaining portion aredirected to the first portion 280.

As a further example, additionally or alternatively, during the timedelay following the first threshold duration, the second mixing valve310 and the first mixing valve 210 are moved to closed positions,thereby fluidly isolating the first 280 and second 282 electrolytecircuits from one another. However, by arranging the second mixing valve310 in the location of the orifice 220 of FIG. 2, the positiveelectrolyte pump 32 may be active during the time delay.

As another example, additionally or alternatively, following the timedelay where the negative electrolyte pump 30 is deactivated and theelectrolytes are directed to flow from the first electrolyte circuit 280to the second electrolyte circuit 282, the second mixing valve 310 ismoved to an open position and the first mixing valve 210 is moved to aclosed position. By moving the second mixing valve 310 to the openposition while the positive electrolyte pump is activated, electrolytesmay flow from the first electrolyte circuit 280 to the secondelectrolyte circuit 282. Electrolytes do not flow from the secondelectrolyte circuit 282 to the first electrolyte circuit 280 via thesecond mixing valve 310.

Thus, the FIGS. 2 and 3 show embodiments for flow battery systems thatutilize the same chemical components in positive and negativeelectrolytes, for example Fe²⁺, salt, and water for an IFB system,further comprising a process to operate these redox flow battery systemsin a cleansing mode, which allows for their operation for an increasednumber of charge and discharge cycles without overall capacity fade ascompared to conventional redox flow battery systems.

Two electrolyte tanks of a redox flow battery system, such as negativeand positive electrolyte chambers 50 and 52 are connected either withone motor controlled mixing valve (e.g., first mixing valve 210) and onecontrol orifice (e.g., orifice 220) or with two motor controlled mixingvalves (e.g., first mixing valve 210 and second mixing valve 310).

When operating the IFB redox flow battery system, battery capacity canbe tracked by monitoring system charge/discharge current and voltage ata given electrolyte SOC. When it is detected that IFB battery capacityis less than a first threshold SOC, for example 90% of a full SOC, acleansing cycle may be triggered.

During operation of the cleansing cycle, the redox flow battery systemmay be automatically placed in discharge mode while rebalancing systemsare on to rapidly reduce all electrolytes' state of charge. Responsiveto a positive electrolyte state of charge is lower than a thresholddischarge SOC and a positive electrolyte pH is lower than the thresholddischarge pH, the first mixing valve 210 is opened to push positiveelectrolyte to the negative side (e.g., first electrolyte circuit 280).Furthermore, in one example, pump 32 may be deactivated to aid inpushing positive electrolyte to the negative side (e.g., firstelectrolyte circuit 280).

In response to the first mixing valve 210 being open for a firstthreshold duration, the first control valve is closed and then mixedelectrolyte in the first electrolyte circuit 280 is directed back topositive electrolyte chamber 52 in the second electrolyte circuit 282 byway of an orifice 220 or by opening a second mixing valve 310. The abovecleansing cycle process may be repeated until a battery state of chargeis lower than a threshold battery SOC, for example 2% of full SOC, afterwhich the redox flow battery system 10 may be ready for operation incharging/discharging mode.

FIG. 4 illustrates an example method 400 for cleansing electrolyte in aredox flow battery system such as the redox flow battery system 10 ofFIG. 1. Instructions for carrying out method 400 (e.g., operating theredox flow battery system in the cleansing mode) and the rest of themethods included herein may be executed by a controller (e.g.,controller 88) based on instructions stored on a non-transitory memoryof the controller and in conjunction with signals received from sensorsof the redox flow battery system 10 such as the sensors described abovewith reference to FIG. 1. The controller may employ actuators of theredox flow battery system to adjust operation thereof, according to themethods described below.

Method 400 begins at 405, where the operating conditions of the redoxflow battery system may be determined. As an example, at 405,electrolyte chemical properties including system charge/discharge modestatus, current, voltage, pH, conductivity, and the like, at a givenelectrolyte state of charge and the like may be measured using varioussensors and/or probes (e.g., sensors 60, 62, 70, 72). At 410, the method400 may include the controller determining if a cleansing cycletriggering condition is met. As one example, the cleansing cycletriggering condition may be met when the redox flow battery capacity isless than a threshold target battery capacity (e.g., threshold SOC). Asan example, the battery state of charge may be determined by opticalscanner, and electrolyte concentration may be monitored using an ORPprobe for measuring electrolyte potential. In one example, the thresholdtarget battery capacity may include 90% of the redox flow batterycapacity. In another example, the threshold target battery capacity mayinclude 97% of the redox flow battery capacity.

Furthermore, the cleansing cycle triggering condition may include whenthe positive electrolyte pH is less than a threshold discharge pH and/orthe positive electrolyte SOC is lower than a threshold discharge SOC. Asdescribed above, the threshold discharge pH may be selected to reduce arisk of Fe(OH)₃ precipitation resulting from mixing the positiveelectrolyte (having a lower pH) and negative electrolyte (having ahigher pH). The threshold discharge SOC may be selected to reduce anamount of Ferric ion being introduced into the negative electrolyte.During discharge of the redox flow battery, ferric ion is reduced toferrous ion (equation (2)), so triggering the cleansing cycle below thethreshold discharge SOC may aid in substantially reducing theconcentration of ferric ion in the positive electrolyte, therebysubstantially reducing a risk of Fe(OH)₃ precipitation. In one example,when the positive electrolyte SOC is below the threshold discharge SOC,the positive electrolyte is essentially free of ferric ion. In oneexample, the threshold discharge pH may include a pH of 1. In anotherexample, the cleansing cycle triggering condition may be met only when abattery SOC is less than the threshold discharge SOC.

If the cleansing cycle trigger condition is met, then the method 400proceeds to 415 where the controller may maintain current batteryoperating conditions and the cleansing cycle is not activated. As such,electrolytes do not flow between the positive and negative electrolytechambers. If the battery capacity is determined to be less than thetarget battery capacity, then the method 400 proceeds from 410 to 420.

At 420, method 400 includes triggering (e.g., activating) the redox flowbattery system cleansing cycle. During the cleansing cycle at 420,rebalancing systems are activated to aid in more quickly reducing theelectrolytes' state of charge. Activating the rebalancing systems mayinclude one or more of the controller directing the electrolyte in thefirst electrolyte circuit 280 through one or more rebalancing reactors80 fluidly connected in series thereto and/or in parallel thereto, andthe controller directing the electrolyte in the second electrolytecircuit 282 through one or more rebalancing reactors 82 fluidlyconnected in series thereto and/or in parallel thereto. In addition, 420includes the controller automatically placing the redox flow batterysystem into discharge mode. In one example, if the battery is in an idlemode prior to 420 and the cleansing cycle triggering condition is met(e.g., the battery state of charge is determined to be less than thethreshold target battery capacity), then the controller switches thebattery to the discharge mode when the cleansing cycle is triggered. Itwill be appreciated that the battery SOC is monitored throughout aplurality of battery modes and/or operating conditions, wherein thecontroller may signal to trigger the cleansing cycle in response to thecleansing cycle triggering condition being met, such as when a batterySOC falls below the threshold target battery capacity.

In some examples, the method may further include determining adifference between the battery SOC and the threshold target batterycapacity. For example, if the battery SOC is 70% of a fully chargedbattery SOC (e.g., 100% SOC) and the threshold target battery capacityis 90% of the full SOC, then the difference measured is 20%. Furtherstill, the cleansing cycle triggering condition may include a differencebetween a battery SOC and a fully charged battery SOC being greater thana threshold SOC difference. In one example, a threshold SOC differencemay include 5%, or may include 3%.

Next, method 400 continues at 430 where both positive electrolyte stateof charge and positive electrolyte pH are determined and compared topre-determined set points. As an example, the positive electrolyte stateof charge may be determined by optical scanner, and the positiveelectrolyte pH may be measured using a pH meter. If the positiveelectrolyte state of charge or positive electrolyte pH are determined tobe greater than their respective pre-determined set points then method400 is directed back to 420 and triggering of the system cleansing cycleis repeated and/or continued. As described above, the positiveelectrolyte pH set point may be selected to reduce a risk of Fe(OH)₃precipitation resulting from mixing the positive electrolyte (having alower pH) and negative electrolyte (having a higher pH). The positiveelectrolyte SOC set point may be selected to reduce an amount of Ferricion being introduced into the negative electrolyte when the positiveelectrolyte is mixed with the negative electrolyte. When both positiveelectrolyte state of charge and pH are determined to be less than theirrespective pre-determined set points then the method 400 continues to440.

At 440, method 400 includes the controller 88 adjusting the positions ofone or more of the mixing valves 210 and 310, and adjusting the pumpstatus of one or more of the electrolyte pumps 30 and 32 to directelectrolyte flow from the second electrolyte circuit 282 to the firstelectrolyte circuit 280. Adjusting the positions of one or more of themixing valves 210 and 310 may include the controller sending a signal toopen motor-controlled mixing valve 210 while activating the electrolytepump 30 to direct flow of (e.g., positive) electrolyte from the secondelectrolyte circuit 282 into the first electrolyte circuit 280 and thenegative electrolyte chamber 50 for a first threshold duration.Furthermore, the positive electrolyte pump 32 may be deactivated,particularly for the case where the redox flow battery system includesthe flow circuit configuration 200 of FIG. 2 including orifice 220. Assuch, electrolytes may not be circulated by the positive electrolytepump 32 through the second electrolyte circuit 282. In this way,electrolytes from the second electrolyte circuit 282 flow to and mixwith electrolytes in the first electrolyte circuit 280 when the firstmixing valve 210 is open and the negative electrolyte pump 30 isactivated. Conversely, while the positive electrolyte pump 32 isdeactivated, the circulating electrolytes in the first electrolytecircuit 280 may not flow to the second electrolyte circuit 282. Thus, at440, the controller may adjust the positions of one or more of themixing valves 210 and 310, and adjust the pump status of one or more ofthe electrolyte pumps 30 and 32 to direct electrolyte flow from thesecond electrolyte circuit 282 to the first electrolyte circuit 280without directing electrolyte flow from the first electrolyte circuit280 to the second electrolyte circuit 282.

In the case where the redox flow battery system includes the flowcircuit configuration 300, adjusting the mixing valve positions andelectrolyte pumps status to direct electrolyte from the secondelectrolyte circuit 282 to the first electrolyte circuit 280 may includeclosing the mixing valve 310 to block flow of electrolyte from the firstelectrolyte circuit 280 to the second electrolyte circuit 282.Furthermore, the electrolyte pump 32 may be activated to maintaincirculation of the electrolyte in the second electrolyte circuit 282while a portion of the electrolyte in the second electrolyte circuit 282flows to the first electrolyte circuit 280 by way of the open mixingvalve 210 and drawn from activated electrolyte pump 30. Because themixing valve 310 is closed while the electrolyte pump 32 is on,electrolyte does not flow from the first electrolyte circuit 280 to thesecond electrolyte circuit 282.

Next, method 400 continues at 442 where the controller determines if afirst threshold duration has elapsed. The first threshold duration maycorrespond to a duration for flowing a sufficient volume of theelectrolyte from the second electrolyte circuit 282 to the firstelectrolyte circuit 280 and mixing thereof at the first electrolytecircuit 280 to maintain a redox flow battery system capacity above thepre-determined threshold target battery capacity. Accordingly, the firstthreshold duration may increase as the difference between the batterySOC and the threshold target battery capacity increases. For example, ifthe threshold target battery capacity is 90%, then the first thresholdduration may be greater when the battery SOC is 70% compared to when thebattery SOC is 80% at the time when the cleansing cycle is triggered.Additionally or alternatively, the first threshold duration may befixed. In one example, the first threshold duration may include 500seconds. If at 442, the controller determines that the first thresholdduration has elapsed, method 400 continues at 444. For the case wherethe controller determines that the first threshold duration has notelapsed, method 400 returns to 440, where the adjusted mixing valvepositions and electrolyte pumps statuses to direct electrolyte from thesecond electrolyte circuit to the first electrolyte circuit aremaintained.

Method 400 continues at 444 where the controller may adjust the mixingvalve positions to stop directing electrolyte from the secondelectrolyte circuit to the first electrolyte circuit. Stopping flow ofelectrolyte from the second electrolyte circuit to the first electrolytecircuit may include the controller closing the mixing valve 210 whilethe electrolyte pump 30 is maintained activated and ON to continuecirculation of electrolyte (including electrolyte directed to the firstelectrolyte circuit 280 from the second electrolyte circuit 282 prior toelapsing of the first threshold duration).

Next, method 400 continues at 446 where the controller determines if atime delay has elapsed. In one example, the time delay includes fixedtime delay. Alternatively, the time delay may be proportionally relatedto the first threshold duration, wherein as the first threshold durationincreases, the time delay increases. In this way, electrolytes flowthrough only the first electrolyte circuit 280 during the time delay.Flowing electrolytes through only the first electrolyte circuit 280during the time delay aids in more thorough mixing of the electrolytesin the first electrolyte circuit 280 during the time delay when nofurther electrolyte flows from the second electrolyte circuit 282 intothe first electrolyte circuit 280. For the case where the time delay hasnot elapsed, method 400 returns to 444 where the adjusted mixing valvepositions to stop flow of electrolyte from the second electrolytecircuit 282 to the first electrolyte circuit 280 are maintained. For thecase where the controller determines that the time delay has elapsed,method 400 continues at 450.

At 450, method 400 includes the controller adjusting one or more of thepositions of mixing valve 210 and 310, and adjusting the electrolytepumps statuses to promote electrolyte flow from the first electrolytecircuit 280 to the second electrolyte circuit 282. Promoting electrolyteflow from the first electrolyte circuit 280 to the second electrolytecircuit 282 may include the controller closing the motor-controlledmixing valve 210, deactivating the electrolyte pump 30, and activatingthe electrolyte pump 32. Activating the electrolyte pump 32 whiledeactivating the electrolyte pump 30 when the mixing valve 210 is closedpromotes drawing of the electrolyte from the first electrolyte circuit280 to the second electrolyte circuit 282. In the case where the redoxflow battery system 10 includes the flow circuit configuration 300,promoting electrolyte flow from the first electrolyte circuit 280 to thesecond electrolyte circuit 282 may further include the controlleropening the motor-controlled mixing valve 310. After mixing valve 210 isclosed, negative electrolyte may be allowed to flow from the firstelectrolyte circuit 280 to the second electrolyte circuit 282, includingelectrolyte flowing from the negative electrolyte chamber 50 to thepositive electrolyte chamber 52 by way of either orifice 220 (in thecase where redox flow battery system 10 includes flow circuitconfiguration 200) or mixing valve 310 (in the case where redox flowbattery system 10 includes flow circuit configuration 300).

In the example of flow circuit configuration 200, step 450 includesflowing electrolyte from the first electrolyte circuit 280 to the secondelectrolyte circuit 282 including the positive electrolyte chamber 52 byway of the orifice 220 (sized for a particular electrolyte mixing rate).In the example of flow circuit configuration 300, step 450 includes thecontroller opening motor-controlled mixing valve 310 to directelectrolyte back to the positive electrolyte chamber 52.

Next, method 400 continues at 456 where the controller determines if asecond threshold duration has elapsed. The second threshold duration maycorrespond to a duration during which a volume of electrolyte can flowfrom the first electrolyte circuit 280 to the second electrolyte circuit282 to balance an electrolyte volume in both of the first and secondelectrolyte circuits 280 and 282. In one example, the second thresholdduration may be equal to the first threshold duration. Alternatively, inthe case where the redox flow battery system 10 include the flow circuitconfiguration 200, the second threshold duration may be based on acombination of a sizing of the orifice 220 and the first thresholdduration. The orifice 220 may be sized to adjust a flow rate from thefirst electrolyte circuit 280 to the second electrolyte circuit 282,and/or the orifice 220 may be sized based on a flow rate of the negativeelectrolyte pump 30. In one example, the orifice 220 is sized to providea flow rate between the first 280 and second 282 electrolyte circuitssubstantially equal to a flow rate of the negative electrolyte pump 30.In one example, a mixing rate between the first 280 and second 282electrolyte circuits increases as a size of the orifice 220 increases.As a diameter (e.g., cross-sectional diameter transverse to main flowdirection) of the orifice increases, a flow rate of electrolyte from thefirst electrolyte circuit 280 to the second electrolyte circuit 282 maybe higher; conversely, as a diameter (e.g., cross-sectional diametertransverse to main flow direction) of the orifice decreases, a flow rateof electrolyte from the first electrolyte circuit 280 to the secondelectrolyte circuit 282 may be lower. When the controller determinesthat the second threshold duration has not elapsed, method 400 returnsto 450 where the controller maintains the adjusted mixing valvepositions and electrolyte pump statuses to promote electrolyte flow fromthe first electrolyte circuit 280 to the second electrolyte circuit 282.For the case where the second threshold duration has elapsed, method 400continues to 460.

Method 400 continues at 460 where the controller determines if a redoxflow battery state of charge is determined and compared to a pre-setvalue, second threshold SOC, SOC_(TH2). For example, if the batterystate of charge is determined by the controller to be higher than thepre-set value, then method 400 returns to step 440 where one or more ofthe mixing valve positions and the electrolyte pumps statuses areadjusted to direct electrolyte flow from the second electrolyte circuit282 to the first electrolyte circuit 280. Thus, electrolyte flow isreversed and electrolytes flow from the positive electrolyte chamber 52to the negative electrolyte chamber 50. At 460, for the case wherebattery state of charge is determined by the controller to be less thanthe pre-set value, SOC_(TH2), then method 400 continues at 470. In oneexample, the pre-set value SOC_(TH2) is less than a threshold lowertarget battery capacity. For example, if the threshold lower targetbattery capacity is 2%, then the pre-set value SOC_(TH2) may be lessthan 2%.

At 470, the controller determines that the redox flow battery system 10is ready for returning to charge/discharge/idle operation as describedwith reference to FIG. 1. As such, in the case where the redox flowbattery system 10 includes the flow circuit configuration 300, thecontroller may adjust a position of the second mixing valve 310 to thefully closed position. Thus, after 470, both the mixing valves 210 and310 are in the fully closed positions and electrolytes are blocked fromflowing between the first electrolyte circuit 280 and the secondelectrolyte circuit 282, including between the positive electrolytechamber 52 and the negative electrolyte chamber 50. Next at 474, thecontroller may end the cleansing cycle and may return operation of theredox flow battery system 10 to charging/discharging/idle mode.Returning operation of the redox flow battery system 10 tocharging/discharging/idle mode may include the controller activating oneor more of electrolyte pumps 30 and 32 to recirculate electrolytebetween the negative and positive electrolyte chambers 50 and 52 and thenegative and positive electrode compartments 20 and 22, respectively.After 474 and 415, method 400 ends.

FIG. 5 is a graph showing the performance comparison of an example redoxiron flow battery (IFB) system including the flow circuit configurationof FIG. 2 or FIG. 3, and operated according to the cleansing methoddescribed herein, versus a typical lithium ion battery. In the figure,percent battery capacity is graphed on the y-axis versus number ofcycles on the x-axis, for both a redox iron flow battery system and alithium ion battery. The graph illustrates that over the courseof >10,000 cycles, the IFB redox flow battery system experiencesvirtually no loss in capacity. In contrast, the lithium ion batterysteadily loses capacity, cycle to cycle, and that after 10,000 cycleshas lost ˜45% of its total capacity. The results depicted in the figureclearly demonstrate the benefits of the cleansing method for an IFBredox flow battery system described herein. The results show that theIFB system is able to operate at full capacity for >10,000 cycles, andthat this system's mitigation of capacity degradation is superior to alithium ion battery system which will have poor operating performanceand therefore require replacement after a significantly fewer number ofcycles.

In this way, the systems and methods described herein may achieve atechnical effect of maintaining increased redox flow battery systemelectrolyte health, including reduced battery system capacitydegradation caused by repeated and cyclic charging and discharging, ascompared with conventional battery systems. In particular, the systemsand methods described herein enable operation of redox flow batterysystems for an increased number of cycles without experiencing a loss ofcapacity greater than a threshold capacity loss. Furthermore, themethods and systems described herein may be performed while utilizingexisting electrolyte storage chambers, and without further additionalelectrolyte storage tanks, thereby reducing a system complexity andcost.

Thus, a method of operating a redox flow battery system includescirculating a positive electrolyte between a positive electrodecompartment and a positive electrolyte chamber with a positiveelectrolyte pump, circulating a negative electrolyte between a negativeelectrode compartment and a negative electrolyte chamber with a negativeelectrolyte pump, and responsive to a first condition, including when aredox flow battery capacity is less than a threshold battery capacity,performing a battery cleansing cycle, including mixing the positiveelectrolyte with the negative electrolyte until a redox flow batterystate of charge (SOC) is less than a threshold SOC. In a first exampleof the method, the first condition further includes when a SOC of thepositive electrolyte is less than a threshold discharge SOC. A secondexample of the method optionally includes the first example and furtherincludes wherein the first condition further includes when a pH of thepositive electrolyte is less than a threshold pH. A third example of themethod optionally includes one or more of the first and second examples,and further includes wherein mixing the positive electrolyte with thenegative electrolyte includes opening a first mixing valve to directflow of the positive electrolyte from the positive electrolyte chamberto the negative electrolyte chamber. A fourth example of the methodoptionally includes one or more of the first through third examples, andfurther includes wherein mixing the positive electrolyte with thenegative electrolyte includes activating the negative electrolyte pumpwhile opening the first mixing valve. A fifth example of the methodoptionally includes one or more of the first through fourth examples,and further includes wherein mixing the positive electrolyte with thenegative electrolyte includes deactivating the positive electrolyte pumpwhile opening the first mixing valve. A sixth example of the methodoptionally includes one or more of the first through fifth examples, andfurther includes wherein mixing the positive electrolyte with thenegative electrolyte includes opening the first mixing valve for a firstthreshold duration. A seventh example of the method optionally includesone or more of the first through sixth examples, and further includeswherein performing the battery cleansing cycle includes switchingoperation of the redox flow battery system to a discharge mode when aSOC of the positive electrolyte is greater than the threshold dischargeSOC. An eighth example of the method optionally includes one or more ofthe first through seventh examples, and further includes whereinperforming the battery cleansing cycle includes directing flow of thepositive electrolyte through a rebalancing reactor when a pH of thepositive electrolyte is greater than the threshold pH. A ninth exampleof the method optionally includes one or more of the first througheighth examples, and further includes, responsive to the first thresholdduration elapsing, closing the first mixing valve and opening a secondmixing valve to direct electrolyte from the negative electrolyte chamberto the positive electrolyte chamber.

Thus, a method of cleansing a redox flow battery system includesoperating the redox flow battery system in a charge, discharge, or idlemode, and responsive to a redox flow battery capacity being less than athreshold battery capacity, switching the redox flow battery system tooperate in the discharge mode, and reducing electrolyte state of charge(SOC) by directing positive and negative electrolytes to flow throughrebalancing reactors. In a first example, the method may include,responsive to a SOC and a pH of the positive electrolyte being less thana threshold positive electrolyte SOC and a threshold pH, respectively,mixing the positive electrolyte with the negative electrolyte. A secondexample of the method optionally includes the first example, and furtherincludes wherein mixing the positive electrolyte with the negativeelectrolyte includes flowing electrolytes from a positive electrodecompartment to a negative electrode compartment for a first thresholdduration. A third example of the method optionally includes one or moreof the first and second examples, and further includes after the firstthreshold duration elapses, flowing electrolytes from the negativeelectrode compartment to the positive electrode compartment for a secondthreshold duration. A fourth example of the method optionally includesone or more of the first through third examples, and further includesfluidly isolating the negative electrode compartment and the positiveelectrode compartment for a time delay following the first thresholdduration and prior to the second threshold duration. A fifth example ofthe method optionally includes one or more of the first through fourthexamples, and further includes wherein the first threshold durationincreases with a difference between the redox flow battery capacity andthe threshold battery capacity.

Thus, a redox flow battery system includes a redox flow battery cellwith a positive electrode compartment and a negative electrodecompartment, a mixing valve fluidly coupled between the positiveelectrode compartment and the negative electrode compartment, positiveand negative electrolyte pumps for circulating electrolyte through thepositive electrode compartment, and a controller, including executableinstructions stored in memory thereon to, responsive to a redox flowbattery capacity being less than a threshold battery capacity, perform abattery cleansing cycle, including mixing the positive electrolyte withthe negative electrolyte. In a first example of the redox flow batterysystem, the executable instructions to mix the positive electrolyte withthe negative electrolyte include opening the mixing valve for a firstthreshold duration while activating the negative electrolyte pump anddeactivating the positive electrolyte pump. A second example of theredox flow battery system includes the first example and furtherincludes wherein the executable instructions further include, after thefirst threshold duration, closing the mixing valve while deactivatingthe negative electrolyte pump and activating the positive electrolytepump. A third example of the redox flow battery system includes one ormore of the first and second examples and further includes wherein theexecutable instructions further include stopping the battery cleansingcycle when a redox flow battery state of charge (SOC) is less than athreshold SOC.

Note that the example control and estimation routines included hereincan be used with various battery configurations. The control methods androutines disclosed herein may be stored as executable instructions innon-transitory memory and may be carried out by the control systemincluding the controller in combination with the various sensors,actuators, and other battery hardware. The specific routines describedherein may represent one or more of any number of processing strategiessuch as event-driven, interrupt-driven, multi-tasking, multi-threading,and the like. As such, various actions, operations, and/or functionsillustrated may be performed in the sequence illustrated, in parallel,or in some cases omitted. Likewise, the order of processing is notnecessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated actions,operations and/or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described actions,operations and/or functions may graphically represent code to beprogrammed into non-transitory memory of the computer readable storagemedium in the engine control system, where the described actions arecarried out by executing the instructions in a system including thevarious battery hardware components in combination with the electroniccontroller.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1-10. (canceled)
 11. A method of cleansing a redox flow battery system,including: operating the redox flow battery system in a charge,discharge, or idle mode, and responsive to a redox flow battery capacitybeing less than a threshold battery capacity, switching the redox flowbattery system to operate in the discharge mode, and reducingelectrolyte state of charge (SOC) by directing positive and negativeelectrolytes to flow through rebalancing reactors.
 12. The method ofclaim 11, further comprising, responsive to a SOC and a pH of thepositive electrolyte being less than a threshold positive electrolyteSOC and a threshold pH, respectively, mixing the positive electrolytewith the negative electrolyte.
 13. The method of claim 12, whereinmixing the positive electrolyte with the negative electrolyte includesflowing electrolytes from a positive electrode compartment to a negativeelectrode compartment for a first threshold duration.
 14. The method ofclaim 13, further comprising, after the first threshold durationelapses, flowing electrolytes from the negative electrode compartment tothe positive electrode compartment for a second threshold duration. 15.The method of claim 14, further comprising fluidly isolating thenegative electrode compartment and the positive electrode compartmentfor a time delay following the first threshold duration and prior to thesecond threshold duration.
 16. The method of claim 15, wherein the firstthreshold duration increases with a difference between the redox flowbattery capacity and the threshold battery capacity.
 17. A redox flowbattery system, including: a redox flow battery cell with a positiveelectrode compartment and a negative electrode compartment; a mixingvalve fluidly coupled between the positive electrode compartment and thenegative electrode compartment; positive and negative electrolyte pumpsfor circulating electrolyte through the positive electrode compartment,and a controller, including executable instructions stored in memorythereon to, responsive to a redox flow battery capacity being less thana threshold battery capacity, perform a battery cleansing cycle,including mixing the positive electrolyte with the negative electrolyte.18. The redox flow battery system of claim 17, wherein the executableinstructions to mix the positive electrolyte with the negativeelectrolyte include opening the mixing valve for a first thresholdduration while activating the negative electrolyte pump and deactivatingthe positive electrolyte pump.
 19. The redox flow battery system ofclaim 18, wherein the executable instructions further include, after thefirst threshold duration, closing the mixing valve while deactivatingthe negative electrolyte pump and activating the positive electrolytepump.
 20. The redox flow battery system of claim 19, wherein theexecutable instructions further include stopping the battery cleansingcycle when a redox flow battery state of charge (SOC) is less than athreshold SOC.